Sulfuric acid is one of the most produced commodity chemicals in the world and is widely used in the chemical industry and commercial products. Generally, production methods involve converting sulphur dioxide first to sulphur trioxide which is then later converted to sulphuric acid. In 1831, P. Phillips developed the contact process which is used to produce most of today's supply of sulphuric acid.
The basics of the contact process involve obtaining a supply of sulphur dioxide (e.g. commonly obtained by burning sulphur or by roasting sulphide minerals) and then oxidizing the sulphur dioxide with oxygen in the presence of a catalyst (typically vanadium oxide) to accelerate the reaction in order to produce sulphur trioxide. The reaction is reversible and exothermic and it is important to appropriately control the temperature of the gases over the catalyst in order to achieve the desired conversion without damaging the contact apparatus which comprises the catalyst.
Then, the produced sulphur trioxide is absorbed into a concentrated sulphuric acid solution to form a higher strength sulfuric acid solution, which is then diluted with water to return the higher strength solution to the original concentration. This avoids the consequences of directly dissolving sulphur trioxide into water which is a highly exothermic reaction.
While the fundamentals of the contact process are relatively simple, it is desirable to maximize the conversion of sulfur dioxide into sulphuric acid and to minimize the emissions of unconverted sulfur dioxide. Thus, modern plants for producing sulphuric acid often involve multiple contact stages and absorption stages to improve conversion and absorption. Further, the plants often involve complex heat exchanger arrangements to improve energy efficiency.
While single contact single absorption (SCSA) systems remain in use, more complex double contact double absorption (DCDA) systems are often employed in order to achieve the ever increasing requirements for higher conversion efficiency and reduced emissions. In a DCDA system, process gases are subjected to two contact and absorption stages in series, (i.e. a first catalytic conversion and subsequent absorption step followed by a second catalytic conversion and absorption step). Details regarding the conventional options available and preferences for sulphuric acid production and the contact process are well known and can be found for instance in “Handbook of Sulfuric Acid Manufacturing”, Douglas Louie, ISBN 0-9738992-0-4, 2005, published by DKL Engineering, Inc., Ontario, Canada.
Platinum catalyst was historically used up to the early 1900s in systems for producing sulfuric acid by the contact process but had certain technical, availability, and economic disadvantages. The platinum catalyst could be poisoned and suffer a loss in activity by the presence of arsenic impurities from roasting sulphide minerals. Over a century ago, the Mannheim process was developed to overcome these problems. In this process, a first conversion stage uses ferric oxide catalyst followed by a SO3 absorption, and then a second conversion stage uses platinum catalyst and a final SO3 absorption. On the economic side however, platinum was and still is relatively rare and expensive.
Platinum was essentially replaced by more economic vanadium oxide catalysts decades ago. And these vanadium oxide catalysts remain as the predominant catalyst choice for the commercial contact process. However, substantial research has been performed towards finding improved catalysts or combinations of catalysts in order to achieve better conversion, reduce cost, and so on.
For instance, U.S. Pat. No. 5,175,136 discloses a process for the manufacture of sulfuric acid in which a gas stream comprising sulfur dioxide and oxygen is passed through a plurality of preliminary contacting stages, in each of which the gas is contacted with a monolithic catalyst comprising a platinum active phase, thereby converting a substantial fraction of the sulfur dioxide in the gas stream to sulfur trioxide. The gas stream leaving one of the plurality of preliminary contacting stages is contacted with sulfuric acid in an absorption zone to remove sulfur trioxide from the stream by absorption in the sulfuric acid. After having passed through the plurality of preliminary stages and the absorption zone, the gas stream is passed through a final contacting stage in which it is contacted with a particulate catalyst comprising vanadium and cesium, thereby substantially converting residual sulfur dioxide in the gas to sulfur trioxide. Platinum was not used at low temperatures and low sulfur dioxide concentrations.
As another example, US2008/0226540 discloses certain ruthenium oxide catalysts that are used in final contact stage for conversion of SO2 to SO3 in multiple stage catalytic converters used in sulfuric acid manufacture. The ruthenium oxide catalysts here provide improved low temperature conversion.
In yet another example, improved emissions using specific combinations of cesium-promoted and conventional vanadium pentoxide catalysts was disclosed in “Optimisation of Anglo Platinum's ACP Acid Plant Catalytic Converter”, M. Sichone, The Southern African Institute of Mining and Metallurgy, Sulphur and Sulphuric Acid Conference 2009.
The contact process can be carried out under adiabatic or isothermal conditions. Most commonly, commercial sulphuric acid plants operate under adiabatic conditions, although isothermal operation can offer potential advantages in principle. GB1504725 for instance discloses a process which may be isothermal for the manufacture of sulfur trioxide, which comprises contacting technically pure sulfur dioxide and oxygen in a tubular heat exchanger in the presence of a suitable catalyst. Nearly pure SO3 can generally be obtained. A catalyst based on vanadium pentoxide is particularly suitable for this process. However, a platinum catalyst and an iron oxide catalyst may also be used. A suitable operating temperature for a V2O5 catalyst is from 420 to 630° C., for a Fe2O3 catalyst from 500 to 780° C. and for a platinum catalyst from 400 to 750° C. If a platinum catalyst is used, those surfaces of the heat exchanger bounding the reaction zone may for example be coated with platinum, a platinum network may be hung into the reaction zone, for example parallel to the axis of the heat exchanger tubes or the reaction zone may be filled with spirally rolled nets. The heat exchanger reaction tube is preferably filled with the catalyst in lump form. Oxidation and heat development occur inside this heat exchanger tube, the heat of the reaction is conducted off directly via the tube walls and consequently the process is isothermal.
Another approach for isothermal or “pseudoisothermal” operation was suggested in U.S. Pat. No. 7,871,593 which discloses a process for the continuous catalytic complete or partial oxidation of a starting gas containing from 0.1 to 66% by volume of sulphur dioxide plus oxygen, in which the catalyst is kept active by means of pseudoisothermal process conditions with introduction or removal of energy. Apparatus for the continuous catalytic complete or partial oxidation of a starting gas containing sulphur dioxide and oxygen having at least one tube contact apparatus is disclosed in the form of an upright heat exchanger composed of at least one double-walled tube whose catalyst-filled inner tube forms a reaction tube. Heat is transferred in cocurrent fashion around the reaction tube using an externally supplied cooling medium (such as air). Objects of the invention were to make possible the inexpensive preparation of sulphuric acid for concentrated starting gases having sulphur dioxide contents of >13.5% by volume and also to provide an economically ecological process for sulphur dioxide-containing off gases from various chemical processes.
Notwithstanding the work done to date in the art, there remains a need for yet further improvements in conversion and energy efficiency, and reductions in emissions and cost in the industrial production of sulphuric acid. The present invention addresses this need and provides other benefits as disclosed below.